Re: [patch -mm 8/9 v2] oom: avoid oom killer for lowmemallocations

[David Rientjes]

On Wed, 17 Feb 2010, KAMEZAWA Hiroyuki wrote:

> > > > I'll add this check to __alloc_pages_may_oom() for the !(gfp_mask & 
> > > > __GFP_NOFAIL) path since we're all content with endlessly looping.
> > > 
> > > Thanks. Yes endlessly looping is far preferable to randomly oopsing
> > > or corrupting memory.
> > > 
> > 
> > Here's the new patch for your consideration.
> > 
> 
> Then, can we take kdump in this endlessly looping situaton ?
> 
> panic_on_oom=always + kdump can do that. 
> 

The endless loop is only helpful if something is going to free memory 
external to the current page allocation: either another task with 
__GFP_WAIT | __GFP_FS that invokes the oom killer, a task that frees 
memory, or a task that exits.

The most notable endless loop in the page allocator is the one when a task 
has been oom killed, gets access to memory reserves, and then cannot find 
a page for a __GFP_NOFAIL allocation:

	do {
		page = get_page_from_freelist(gfp_mask, nodemask, order,
			zonelist, high_zoneidx, ALLOC_NO_WATERMARKS,
			preferred_zone, migratetype);

		if (!page && gfp_mask & __GFP_NOFAIL)
			congestion_wait(BLK_RW_ASYNC, HZ/50);
	} while (!page && (gfp_mask & __GFP_NOFAIL));

We don't expect any such allocations to happen during the exit path, but 
we could probably find some in the fs layer.

I don't want to check sysctl_panic_on_oom in the page allocator because it 
would start panicking the machine unnecessarily for the integrity 
metadata GFP_NOIO | __GFP_NOFAIL allocation, for any 
order > PAGE_ALLOC_COSTLY_ORDER, or for users who can't lock the zonelist 
for oom kill that wouldn't have panicked before.
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